When a plastic material is used in a useful article, such as a container, it is subjected to many types of stress. Stress can be caused by external factors, by deforming forces applied to the material, or by strain which remains in the material from the time of its fabrication. Additionally, the environment to which the material is exposed, i.e. gaseous or liquid, can worsen the effects of stress on the material. This latter phenomenon is environmental stress cracking.
Certain environmental factors cause materials, e.g., plastic articles, to exhibit failure, generally by cracking, at stesses lower than those which cause cracking in the material under ideal (normal) circumstances. Crazing is a precursor to environmentally induced stress cracking. Generally, crazing is caused by the reaction of the material to the environmental factors, in combination with stresses existing within the material or which are applied to the material.
Environmental stress cracking is often the life limiting mode of failure for polymer systems under stress, such as in the case with blow-molded polyethylene terephthalate (PET) beverage containers. Therefore, it is important in the production and use of plastic articles to minimize environmental stress cracking. The design of a plastic article is particularly important where the end use or handling of such article is likely to involve contact with materials known to promote environmental stress cracking.
Blow molded PET containers are widely used in the beverage industry. Particularly well-known are two-liter containers formed of blow molded PET, which serve as containers for beverages of all kinds, e.g., soft drinks. It is especially important to minimize environmental stress cracking for such containers. Major soft drink manufacturers and the ultimate consumers desire containers which will be free, or substantially free, of the tendency to develop environmental stress cracking.
Heretofore PET containers, such as the aforementioned containers for soft drinks, have been constructed in a two-liter configuration. The bottle portion thereof has been blow molded such that there is a narrow neck having an upper mouth, a rounded (e.g., hemispherical) base and a cylindrical sidewall therebetween. Inasmuch as containers having a rounded base cannot stand upright, containers such as these often include a boot or base platform into which the container fits. The boot is typically made of a different kind or grade of plastic.
Containers having two distinct pieces are undesirable from a manufacturing standpoint. Moreover, recycling of two-piece bottles is more difficult than for one-piece containers. It is therefore desirable to have one-piece containers which are capable of standing, without the need for a boot.
A one-piece container requires that the base thereof be molded in a manner that permits it to stand on its own. Such molding, however, tends to increase the susceptibility to crazing and eventual failure. While crazing is not as prevalent in those portions of plastic articles which have been stretched (due to the highly oriented nature of the molecules in those regions), crazing is more likely to occur in areas where there is no noticeable or substantial orientation, such as are caused by the molding of the bottom portion of stand-alone PET containers.
When containers of this type are filled with soft drinks, they are typically pressurized to between about 60 and about 65 psi. Increases in temperature can cause the internal pressure to, at times, reach 90 to 95 psi or more. Such pressure not only affects initiation of crazing, but also affects distortion of the container shape. Again, such distortions are typically noted in areas where the plastic has been unoriented, i.e., particularly at or close to the center of the container base. This pressure-induced distortion also gives rise to crazing, and eventual cracking of the bottle. In particular, the more the bottle is distorted, the greater the possibility of crazing and eventual failure. In general, for PET articles such as sheets, 5% to 7% elongation is a critical point for craze initiation. It is not easy to determine the amount of localized elongation (or strain) in formed articles, such as the one-piece, stand-alone containers described above.
In addition to stress level and environment, there are several factors which can affect the environmental stress crack resistance of PET containers. These factors include, among others, processing conditions and container design. Inasmuch as the environments which cause crazing cannot always be avoided, materials, process conditions, and especially container design must be optimized with environmental stress crack resistance in mind.
Known methods for determining the tendency of plastic articles to undergo environmental stress cracking include tensile load methods, such as the so called Lander Test (ASTM-D-2552). Other standard tests include ASTM-D-1963-60T, wherein a test sample is nicked with a razor blade, bent into a U-shaped configuration and submerged into a solution of a stress cracking agent.
Methods have also been described for determining the environmental stress crack resistance of polymer articles. These involve the external application of stress to an unstressed test article which has been exposed to a stress cracking agent, such as by means of a weight. See, Fisher, et al., U.S. Pat. No. 4,829,839 issued May 16, 1989. Other methods describe the use of a weighted rod which stresses a plastic article in contact with a stress cracking agent in an electrically conductive liquid until the liquid penetrates the stress cracks. See, Smith, et al., U.S. Pat. No. 3,710,616 issued Jan. 16, 1973. Another method includes initially deforming a test piece in a test environment followed by the application of additional stress of a magnitude smaller than that of the stress already produced in the test piece, until the test piece fractures. See, Saeda, et al., U.S. Pat. No. 4,107,979 issued Aug. 22, 1978. These methods are unsuitable for determining the environmental stress crack resistance of blow molded PET containers.
Heretofore such containers have been tested by filling the container with a carbonated beverage under an internal pressure, exposing the container to a line lubricant or other environmental stress cracking agent, and then measuring the time it takes for the container to explode or otherwise show signs of deformation. It is also known to pressurize the container, expose it to acetone or another environmental stress cracking agent, and then pressurize the container until failure occurs.
ASTM-D-2561-70, entitled "Standard Test Methods For Environmental Stress-Cracked Resistance Of Blow-Molded Polyethylene Containers", discloses three procedures for measuring the environmental stress crack resistance of containers made from polyethylene and subjected to an ESC agent, internally in most cases. Each of the procedures is continued until failure, defined as "the formation of any imperfection, such as a crack, which results in a loss of pressurizing gas or stress cracking agent" is observed. These tests are unsuitable for PET containers because, in the test procedure, the container is subjected to the test method until the container has fractured. To fully measure the environmental stress crack resistance of PET containers, it is necessary to collect data on the performance of the container under conditions which exist well before the container fractures. Particularly, preventing conditions which could eventually lead to a fracture is important in the container industry, and ASTM-D-2561-70 does not address that problem.
While some information can be obtained by subjecting blow molded PET containers to the foregoing tests, the tests are unsuitable inasmuch as the results are often difficult to interpret or take too long to obtain. The way such tests are carried out also does not provide information that can be directly correlated to actual field failures. Moreover, the foregoing tests fail to appreciate the phenomenon of crazing, and thus fail to appreciate the significance of information and data which can be obtained when crazing begins.